The invention relates to uses of ceramics in dental and orthodontic applications.
Although performance and durability are highly desirable characteristics for dental replacement and repair work, for example, they alone are not the sole concern for practitioners and patients. Aesthetic value, or how dental materials and articles and orthodontic appliances look inside the mouth is just as desirable.
For example, in prosthodontics and restorative dentistry, where tooth replacement or prostheses are custom made to fit in or on a tooth structure, there are instances where the restoration or repair can be seen from a short distance when the mouth is open. Thus in those instances, it would be highly desired that the dental material be nearly indistinguishable from adjacent tooth structure.
Prosthetics and restorative dentistry encompass the fabrication and installation of, for example, restoratives, replacements, inlays, onlays, veneers, full and partial crowns, bridges, implants, and posts. Conventional materials used to make dental prostheses include gold, ceramics, amalgam, porcelain, and composites. In terms of aesthetic value, it is perceived that porcelains, composites, and ceramics look better than amalgam and metals, since a prosthetic made from those nonmetals better matches or blends in with the color of adjacent natural teeth.
For orthodontic appliances (typically, brackets, which are small slotted bodies for holding a curved arch wire, and associated tooth bands if banded attachment is used), stainless steel is an ideal material because it is strong, nonabsorbent, weldable, and relatively easy to form and machine. A significant drawback of metal appliances, however, relates to cosmetic appearance when the patient smiles. Adults and older children undergoing orthodontic treatment are often embarrassed by the “metallic smile” appearance of metal bands and brackets, and this problem has led to various improvements in recent years.
One area of improvement involves use of nonmetal materials. Both plastic and ceramic materials present an improved appearance in the mouth, and often the only significantly visible metal components are thin arch wires that are cosmetically acceptable. Plastic is not an ideal material because it lacks the structural strength of metal, and is susceptible to staining and other problems. Ceramics such as sapphire or other transparent crystalline materials have undesirable prismatic effects. Also, single crystal aluminum oxide appliances are subject to cleavage under the loads that occur in the course of orthodontic treatment. Other ceramics have been largely opaque so that they either do not match tooth color or require coloring.
Glasses and glass-ceramics have also been used for dental replacement and repair work. Sinterable glass-ceramics based on lithium disilicate utilized in production of shaped dental products are known. For example, some compositions are based on SiO2 (57-80 wt-%) and Li2O (11-19 wt-%) with minor amounts of Al2O3, La2O3, MgO, ZnO, K2O, P2O5 and other materials. Another examples are moldable ceramic-glass compositions which include 50-99 parts by weight of alumina and/or zirconia powder and 1 to 50 parts by weight of glass powder.
Digitized machining of ceramics (commonly known as CAD/CAM milling) is one method for producing useful dental shapes. However, the machining of fully densified structural ceramics like Al2O3 and ZrO2 into complex dental geometries is difficult due to rapid tool wear. For this reason, methods involving machining of green ceramic body have been developed (e.g. LAVA ZrO2 by 3M Company).